In general, thermodynamics is the science that deals with energy production, storage, transfer and conversion. Currently, fossil fuel is still the world’s main energy source. But the burning of fossil fuels generates only thermal energy, therefore these energy sources are also called as primary energy sources. This type of energy must be converted to secondary energy source, also called as energy carriers, such as electrical energy. To convert thermal energy into another form of energy a heat engine must be used.
Many heat engines operate in a cycle, which means that adding energy in the form of heat occurs in one part of the cycle and using that energy to do useful work occurs in another part of the cycle. A process that eventually returns a system to its initial state is called a cyclic process. At the conclusion of a cycle, all the properties have the same value they had at the beginning of the cycle. Typical thermodynamic cycle consists of a series of thermodynamic processes transferring heat and work, while pressure, temperature, and other state variables vary. Eventually it returns a system to its initial state where it started.
The thermodynamic cycles can be divided into two primary classes:
- Power cycles: Power cycles are cycles which convert some heat input into a mechanical work output. Thermodynamic power cycles are the basis for the operation of heat engines, which run the majority of motor vehicles and generate most of the world’s electric power.
- Heat pump cycles: Heat pump cycles transfer heat from low to high temperatures using mechanical work input. There is no difference between thermodynamics of refrigerators and heat pumps. Both work by moving heat from a cold space to a warm space.
In practice, simple idealized thermodynamic cycles are usually made out of four thermodynamic processes. In general, the following processes usually constitute thermodynamic cycles:
- Adiabatic process
- Isothermal process
- Isobaric process
- Isochoric process
In practice, simple idealized thermodynamic cycles are usually made out of four thermodynamic processes. Any thermodynamic processes can be used. However, when idealized cycles are modeled, often processes where one state variable is kept constant are used, such as an isothermal process (constant temperature), isobaric process (constant pressure), isochoric process (constant volume), isentropic process (constant entropy), or an isenthalpic process (constant enthalpy). Often adiabatic processes are also used, where no heat is exchanged.
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| Ideal cycle |
An ideal cycle is constructed out of:
1 to 2 and 3 to 4: a pair of parallel isobaric processes
2 to 3 and 4 to 1: a pair of parallel isochoric processes
Internal energy of a perfect gas undergoing different portions of a cycle:
In 1824, a French engineer and physicist, Nicolas Léonard Sadi Carnot enhanced the study of the second thermodynamic law by forming a principle that specifies limits on the maximum efficiency any heat engine can obtain. In simple words, this principle states that the efficiency of a thermodynamic cycle depends solely on the difference between the hot and cold temperature reservoirs.
Carnot’s principle states:
- No engine can be more efficient than a reversible engine (a Carnot heat engine) operating between the same high temperature and low temperature reservoirs.
- The efficiencies of all reversible engines (Carnot heat engines) operating between the same constant temperature reservoirs are the same, regardless of the working substance employed or the operation details.
The cycle of this engine is called the Carnot cycle. A system undergoing a Carnot cycle is called a Carnot heat engine. It is not an actual thermodynamic cycle but it is a theoretical cycle and cannot be built in real life. All real thermodynamic processes are somehow irreversible. They are not done infinitely slowly. Infinitesimally small steps in temperature are also only a theoretical construct. Thus, heat engines must have lower efficiencies than limits on their efficiency due to the inherent irreversibility of the heat engine cycle they use.
To move the rocket, we need to create a thrust. Thrust is generated by the propulsion system. A propulsion system is a machine that produces thrust to push an object forward. In a rocket engine, fuel and a source of oxygen, called an oxidizer, are mixed and exploded in a combustion chamber. The combustion produces hot exhaust which is passed through a nozzle to accelerate the flow and produce thrust. For a rocket, the accelerated gas, or working fluid, is the hot exhaust produced during combustion. This is a different working fluid than you find in a turbine engine or a propeller powered aircraft. Turbine engines and propellers use air from the atmosphere around as the working fluid, but rockets use the combustion exhaust gases. In the space there is no atmosphere so turbines and propellers cannot work there.
There are two main categories of rocket engines; liquid rockets and solid rockets.
In a liquid rocket, the propellants, the fuel and the oxidizer, are stored separately as liquids and are pumped into the combustion chamber of the nozzle where burning occurs.
In a solid rocket, the propellants are mixed together and packed into a solid cylinder. Under normal temperature conditions, the propellants do not burn; but they will burn when exposed to a source of heat provided by an igniter. Once the burning starts, it proceeds until all the propellant is exhausted. With a liquid rocket, you can stop the thrust by turning off the flow of propellants; but with a solid rocket, you have to destroy the casing to stop the engine. Liquid rockets tend to be heavier and more complex because of the pumps and storage tanks. The propellants are loaded into the rocket just before launch. A solid rocket is much easier to handle and can sit for years before firing.
In solid and liquid fueled rocket engines, the working gas is produced through the burning of a fuel to produce power. Burning a fuel is called combustion. Combustion is a chemical process in which a substance reacts rapidly with oxygen and gives off heat. Combustion needs three things to occur: a fuel to be burned, a source of oxygen, and a source of heat. As a result of combustion, exhausts are created and heat is released. It is possible to control or stop the combustion process by controlling the amount of the fuel available, the amount of oxygen available, or the source of the heat.
For liquid-propellant rockets, four different ways of powering the injection of the propellant into the chamber are in common use.
Fuel and oxidizer must be pumped into the combustion chamber against the pressure of the hot gasses being burned, and engine power is limited by the rate at which propellant can be pumped into the combustion chamber. For atmospheric or launcher use, high pressure, and thus high power, engine cycles are desirable to minimize gravity drag. For orbital use, lower power cycles are usually used.
Pressure-fed cycle
The propellants are forced in from pressurized (relatively heavy) tanks. The heavy tanks mean that a relatively low pressure is optimal, limiting engine power, but all the fuel is burned, allowing high efficiency. The fuel used is frequently helium due to its lack of reactivity and low density.
Examples: AJ-10, used in the Space Shuttle OMS, Apollo SPS, and the second stage of the Delta II.
Gas-generator cycle
A small percentage of the propellants are burnt in a pre-burner to power a turbopump and then exhausted through a separate nozzle, or low down on the main one. This results in a reduction in efficiency since the exhaust contributes little or no thrust, but the pump turbines can be very large, allowing for high power engines.
Examples: Saturn V's F-1 and J-2, Delta IV's RS-68, Ariane 5's HM7B, Falcon 9's Merlin.
Expander cycle
Cryogenic fuel (hydrogen, or methane) is used to cool the walls of the combustion chamber and nozzle. Absorbed heat vaporizes and expands the fuel which is then used to drive the turbopumps before it enters the combustion chamber, allowing for high efficiency, or is bled overboard, allowing for higher power turbopumps. The limited heat available to vaporize the fuel constrains engine power.
Examples: RL10 for Atlas V and Delta IV second stages (closed cycle), H-II's LE-5 (bleed cycle).
Staged combustion cycle
A fuel- or oxidizer-rich mixture is burned in a preburner and then drives turbopumps, and this high-pressure exhaust is fed directly into the main chamber where the remainder of the fuel or oxidizer undergoes combustion, permitting very high pressures and efficiency.
Examples: SSME, RD-191, LE-7.
Full-flow staged combustion cycle
Fuel- and oxidizer-rich mixtures are burned in separate preburners and driving the turbopumps, then both high-pressure exhausts, one oxygen rich and the other fuel rich, are fed directly into the main chamber where they combine and combust, permitting very high pressures and incredible efficiency.
Example: SpaceX Raptor.
SpaceX Raptor
Raptor is a family of full-flow staged combustion cycle rocket engines developed and manufactured by SpaceX, for use on the in-development Starship fully reusable launch vehicle. The engine is powered by cryogenic liquid methane and liquid oxygen (LOX), rather than the RP-1 kerosene and LOX used in SpaceX's prior Merlin and Kestrel rocket engines. The Raptor engine has more than twice the thrust of SpaceX's Merlin engine that powers their current Falcon 9 and Falcon Heavy launch vehicles.
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| Source Wiki: SpaceX Raptor FFSC rocket engine, sample propellant flow schematic, 2019 |
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